An Engineer's Ultimate Guide To 3.21 VS 3.92 Axle Ratio

[Rebelguy2020]

OEM size tires are all basically the same height. There won’t be any noticeable RPM difference between any of them.

275/65/18 = 32.08”
275/55/20 = 31.90”
285/45/22 = 32.10”

Rebels have the 275/70R18=33” tires so the rpms drops slightly with the 3.92.
Just completed a 2000km trip, 1400km was towing a loaded (one way) 8x16 enclosed trailer and very happy with my 3.92. The best thing is that I didn’t even burn one drop of gas, but got 21 mpg (imperial) of diesel, hand calculated, cruising average of 55 mph. I would have done better but I was fighting strong crosswinds on the way back, but the trailer empty.
The small displacement 3 litre diesel has 480 ft/lb of torque at 1600 rpm which is very close to the 55 mph cruising speed that I was traveling with the trailer.
I absolutely don’t miss my Hemi, the Hemi sure had fast pulling power but at 4500 rpm or higher to get to 410 ft/lb of torque.

 

[BowDown]

If there was significantly more parasitic loss then yes it would, but there isn’t. That was debunked years (10+) ago on Corvetteforum.

It’s been a long time since I’ve read it but basically it was found that the difference is in relation to how the dyno calculates the time it takes the engine to rev from A to B. And within that calculation it ends up with an estimate that will show lower power.

Debunked by who? RPM transmissions says it's real as do most big tuners. It's not a huge difference but it's been proven 4-14 whp difference depending on the gear.

 

[BowDown]

Sure but mpg has fluctuations, it’s not a straight 6 mpg drop

400+ miles with mixed driving is achievable with 3.92, RPM’s are 22% higher, so if your at 1400 I’m at 1700

View attachment 96557

How much of thst is highway?

 

[BowDown]

Rebels have the 275/70R18=33” tires so the rpms drops slightly with the 3.92.
Just completed a 2000km trip, 1400km was towing a loaded (one way) 8x16 enclosed trailer and very happy with my 3.92. The best thing is that I didn’t even burn one drop of gas, but got 21 mpg (imperial) of diesel, hand calculated, cruising average of 55 mph. I would have done better but I was fighting strong crosswinds on the way back, but the trailer empty.
The small displacement 3 litre diesel has 480 ft/lb of torque at 1600 rpm which is very close to the 55 mph cruising speed that I was traveling with the trailer.
I absolutely don’t miss my Hemi, the Hemi sure had fast pulling power but at 4500 rpm or higher to get to 410 ft/lb of torque.

Ain't nobody talking to you 30 mpg gettin diesel guys lol.

 

[Rebelguy2020]

Ain't nobody talking to you 30 mpg gettin diesel guys lol.

Sorry, that was kind of a low blow in my part, but it feels good to brag sometimes

 

[Mountain Whiskey]

This is a crazy debate of theories and calculations that only account for a couple of factors. It is not only about rpm. It is about the other modifications to your truck. Larger tires and lifts do more than change rpms they change the load placed on the motor.

When I put 33s on a jeep tj, the gas milage dropped like a rock. Put new gears in it and the milage jumped back up. Sure, higher rpm but less strain to maintain momentum (in a vehicle less aerodynamic than a cow).

Driving habits, tire size and weight, terrain, speed (by the way, 70+ mph is not helping anyone's milage), accessories and much more affect the milage. If you want good mileage you could be driving one of these little death traps...


Just accept the known fact that the 3.92 gear ratio is superior in our trucks and it will all be OK.

Accept it.

 

[HSKR R/T]

Just got back from weekend trip to Oklahoma City area from Omaha, NE. Averaged 16mpg on trip down, actual calculated mileage, not going off trip computer. First tank on way back, trip computer showed 17.3mpg, calculated out to 16.3mpg. final leg or trip, trip computer shows what is in picture. Will fill up tomorrow and do actual calculation. This is 5.7 Built to Serve with ORP, 4x4. Borla ATAK, Vararam, and Pulsar set to "economy" with 91 Oct fuel.

 

[theblet]

With the 3.21s I’m constantly in 8th gear above 50 mph. With the cruise set I have mds until about 65. Then it’s iffy depending on the wind.

 

[IvoryHemi]

How much of thst is highway?

3 of those 12 hours were highway.

 

[silver billet]

This is a crazy debate of theories and calculations that only account for a couple of factors. It is not only about rpm. It is about the other modifications to your truck. Larger tires and lifts do more than change rpms they change the load placed on the motor.

When I put 33s on a jeep tj, the gas milage dropped like a rock. Put new gears in it and the milage jumped back up. Sure, higher rpm but less strain to maintain momentum (in a vehicle less aerodynamic than a cow).

Driving habits, tire size and weight, terrain, speed (by the way, 70+ mph is not helping anyone's milage), accessories and much more affect the milage. If you want good mileage you could be driving one of these little death traps...
View attachment 96578

Just accept the known fact that the 3.92 gear ratio is superior in our trucks and it will all be OK.

Accept it.

I've accepted that my truck is not a 2500/cummins and due to payload in all our trucks, can only tow 8000 pounds; which means the 3.92 is completely useless unless you A) offroad and/or B) run larger tires, neither of which interest me. I prefer to save money on gas, and cruise at 500 RPM's less, thanks.

Nobody is saying what you said above; we're simply saying, if two trucks are otherwise identical and driven the same way by the same person, that 400 to 500 RPM's is going to cost you at least 3+ MPG on the freeway. We know, because we can compare instant readout when we drop out 3.21's into 7th and compare what it was at in 8th.

That my friend, is a fact. The rest of your post is just smoke and mirrors.

 

[BowDown]

I've accepted that my truck is not a 2500/cummins and due to payload in all our trucks, can only tow 8000 pounds; which means the 3.92 is completely useless unless you A) offroad and/or B) run larger tires, neither of which interest me. I prefer to save money on gas, and cruise at 500 RPM's less, thanks.

Nobody is saying what you said above; we're simply saying, if two trucks are otherwise identical and driven the same way by the same person, that 400 to 500 RPM's is going to cost you at least 3+ MPG on the freeway. We know, because we can compare instant readout when we drop out 3.21's into 7th and compare what it was at in 8th.

That my friend, is a fact. The rest of your post is just smoke and mirrors.

Exactly.
I bought this truck in this configuration for luxury and still have a truck for truck use but get the best mileage I can get; I've had a 3.92 truck, never again.

i also have no desire for big tires or off-roading, I occasionally tow my car to the track, aside from that, I don't need towing and a 3.92 truck isn't fast nor is a 3.21 truck, for that matter, if I want fast, I'll get in the car built for that.

I keep reading posts about poor fuel mileage yet almost all these trucks are 4x4 and 3.92. I'm happy with my mileage especially given that it's mostly city driving

 

[mmcbeat]

The “fun factor” that comes with the 3.92 seems to be overlooked. I’m not talking burn outs, power stalls, and donuts. Just safe spirited driving. I don’t drive a lot and most of my trips are short but I enjoy the highway on ramps, getting up to speed on the freeways, etc.
…..key phrase, “I don’t drive a lot” so I’m not too concerned about mileage.

 

[IvoryHemi]

Most people aren’t driving 80-100 mph. So having the engine at 400-500 rpm higher really isn’t a factor

 

[Cliffrunner88]

I've accepted that my truck is not a 2500/cummins and due to payload in all our trucks, can only tow 8000 pounds; which means the 3.92 is completely useless unless you A) offroad and/or B) run larger tires, neither of which interest me. I prefer to save money on gas, and cruise at 500 RPM's less, thanks.

Nobody is saying what you said above; we're simply saying, if two trucks are otherwise identical and driven the same way by the same person, that 400 to 500 RPM's is going to cost you at least 3+ MPG on the freeway. We know, because we can compare instant readout when we drop out 3.21's into 7th and compare what it was at in 8th.

That my friend, is a fact. The rest of your post is just smoke and mirrors.

The “fun factor” that comes with the 3.92 seems to be overlooked. I’m not talking burn outs, power stalls, and donuts. Just safe spirited driving. I don’t drive a lot and most of my trips are short but I enjoy the highway on ramps, getting up to speed on the freeways, etc.
…..key phrase, “I don’t drive a lot” so I’m not too concerned about mileage.

Agree with both. I figure that the load I am "towing" is my truck itself. It will be pretty loaded and therefore heavy, so having the 3.92 will help me move my fat ***, lol. And I don't do a lot of highway driving (some on occasion sure but) so the MPGs should be VERY similar at speeds less than 60mph.

 

[theblet]

I wanted the 3.21 for the fuel savings, as I don’t tow often or drag race. Has more than enough get up and go for me.

 

[Cliffrunner88]

I wanted the 3.21 for the fuel savings, as I don’t tow often or drag race. Has more than enough get up and go for me.

I originally had 3.21 for your same reasons but changed my build. Trying to find an apples to apples comparison is nearly impossible in my little town.

 

[brian42]

I could have gone either way but the only truck I wanted had the 3.21 (I only had 2 weeks to buy something off the lot so ordering one was not an option).

I drove both 3.21 and 3.92 in similar trucks. I really didn't notice a difference unless I hammered the go pedal. I'm sure the lower first gear probably took off a hair quicker at partial throttle but my SOTP dyno didn't notice anything to make it a deal-breaker.

Since I didn't plan on driving my RAM like it was an SRT Hellcat I'm happy with my choice to "live with" the 3.21 rear.

If I had any aspirations of larger/heavier tires or towing I would have made the 3.92 a "must have".

Just my .02

 

[BowDown]

The “fun factor” that comes with the 3.92 seems to be overlooked. I’m not talking burn outs, power stalls, and donuts. Just safe spirited driving. I don’t drive a lot and most of my trips are short but I enjoy the highway on ramps, getting up to speed on the freeways, etc.
…..key phrase, “I don’t drive a lot” so I’m not too concerned about mileage.

While I understand that for some people, I have a low 10 sec N/A 730hp Z06. After driving that car, nothing about this truck is spirited, matter of fact, I get in the truck and I want to hear nothing, no exhaust, no cold air intake noise, nothing.

 

[Mountain Whiskey]

Yup. I'm convinced. I'm trading her I for a new truck with 3:21 gears and keeping it stock.

Then I can put this sticker in the rear window and ride around in the left lane.

 

[Rossi396]

The “fun factor” that comes with the 3.92 seems to be overlooked. I’m not talking burn outs, power stalls, and donuts. Just safe spirited driving. I don’t drive a lot and most of my trips are short but I enjoy the highway on ramps, getting up to speed on the freeways, etc.
…..key phrase, “I don’t drive a lot” so I’m not too concerned about mileage.

“This!” I drove a 3.21 and. 3.92 back-to-back and I absolutely felt a difference in the butt dyno. I do tow, so that was part of the reason I chose the 3.92, but what amazes me is I get better mileage out of my new Hemi than I did out of any of my previous THREE Tacoma’s. On a recent trip from Pittsburgh to the WV mountains, I averaged 20.2 MPG for a little over 300 miles. I didn’t buy the truck for the mileage but I’m ecstatic with that. And for the record, I’ve driven a high “8-second” Chevy II chassis car and I still enjoy driving this truck! I find myself taking it on a trip to the post office or Lowe’s instead of my Accord. Whatever rear end you have, enjoy your truck!!